This invention relates, in general, to electrical interconnect, and more particularly to solder ball interconnect.
It is well known by one skilled in the art that solder ball or bump technology is commonly used for electrical and mechanical interconnection of an integrated circuit to a substrate. In general, solder balls are formed on pads of either the integrated circuit, the substrate, or both. The integrated circuit and substrate are aligned and mated together, the assembly is then heated to reflow the solder balls coupling and electrically connecting the integrated circuit to the substrate. The initial formation of a solder ball on a contact pad is critical. Issues such as solder ball uniformity, conductive quality, bond strength, and ball height are critical to producing reliable interconnection. Increasing solder ball height while decreasing pad pitch (for more interconnect) is extremely difficult for many solder ball manufacturing methods. Solder balls are generally formed of lead and tin. High concentrations of lead are sometimes used to make the bump more compatible to subsequent processing steps. Tin is added to strengthen bonding (to such metals as copper) and serves as an anti-oxidant.
One method well known in the industry for forming solder balls is using a metal plating process. Pad areas are isolated by photoresist and metal is deposited, the composition of which forms solder when reflowed. The photoresist height surrounding the pad area limits the amount of metal being deposited. The well or via formed by the photoresist on the pad area is filled with metal. The photoresist is then removed and the metal on the pad areas is reflowed forming a solder ball. Solder balls are formed having a height ranging from 50 to 150 microns. Two problems areas exist for this method when the height of the bump is increased and the pad pitch is decreased. First, it is difficult to form photoresist accurately when applied very thick. Multiple steps can be used to "build" the photoresist up at the expense of increased manufacturing steps. Second, as the well or via height increases, great care must be taken to insure that metal is distributed evenly throughout the wafer so that the solder balls are formed having a uniform height.
A second method well known in the industry for forming solder balls is using a metal evaporative process. Pad areas are isolated by the use of a mask, often made of metal, the various metals are deposited through the vias formed in the mask by condensation of the evaporated metals on the substrate. The final composition and shape of the solder balls is usually achieved after the metal is reflowed. The size and height of the solder balls is determined by the vias formed in the mask and the thickness of the mask. The major problem with this methodology is the high cost for the evaporators and the possible high cost for the metal masks. Another problem can be the temperature rise of the mask and substrate caused by the condensation of the evaporated metal on the substrate and mask. Since the substrate and mask are usually made of different materials, their coefficient of thermal expansion is also different. The coefficient of thermal expansion mismatch causes problems with alignment of the via to pad on large substrates at elevated temperatures. The mask must also be cleaned after each deposition, because in an evaporator, the metal is equally distributed over the entire surface of the mask and via.
A third solder ball method used in the industry is preformed solder ball placement and reflow technique. Preformed solder balls are manufactured of a uniform size. The solder balls are then placed on the pad areas of an integrated circuit or substrate. The equipment to perform this task is complex and expensive. Once aligned to the pad areas, the preformed solder balls are reflowed to connect to the pad area. This technique is not good for tight pad pitches and small solder balls.
It would be of great benefit if a method for making interconnect could be developed which simplified manufacturing, yet formed interconnect of increased height and density.